It is previously known to change the properties of wood products by pressure treatment. SE 446 702 describes one method for hardening and stabilizing wood. The method comprises heating the wood to a temperature of 75-160.degree. C. and compressing it by mechanical pressing at a compression pressure of 50-1000 bar for 0.1-60 seconds. For carrying out this treatment, a roller press is used. However, it has proved that this method leads to an uneven treatment result. The change in hardness of the treated wood varies markedly between various points on the treated surface. This insufficient treatment result is primarily connected with disadvantages of the roller press. During calendering, the treatment pressure is applied along a line on the surface of the treated element. The calendering means that small natural variations in the thickness and density of the untreated wood element result in different local roll pressures. Thus, also the treatment result varies locally. Furthermore, calendering results in the pressure being applied in only one direction. During the rolling, therefore, the treated element tends to float out in breadth and in length. This results, among other things, in the edges of the element becoming uneven. Still worse, however, is that the unidirectional pressure contributes to the locally varying treatment result. The material in those parts of the element which are located along the edges of the element easily floats out and only experiences the vertical compression. In those parts which are not situated at the edge, on the other hand, the internal friction of the wood prevents the material from floating out. The material in these parts therefore experiences also a certain and greatly varying internal horizontal counter-pressure, which leads to making these parts harder than at the edges of the element. It is primarily the locally varying hardness result that makes calendering unsuitable for pressure treatment of wood elements.
Swedish patent application 7805483-0 describes a method for pressing veneer. The description states that it is known to press veneer at moderate pressures of about 1-5 MPa and also that it has proved to be suitable, when pressing pine wood veneer, to work with pressures around 150-350 MPa. According to this document, the pressure should be applied by means of a hydraulic press across the whole veneer surface simultaneously. These statements would seem to be unreasonable to a person skilled in the art. Generating such high pressures over such large surfaces with prior art technique would seem to be, if not impossible, at least commercially unrealizable. It is more likely that the pressures intended throughout the description are to be given in the unit bar, that is, that the actual pressures intended are one-tenth of those given in MPa in the description.
With this interpretation, the description becomes more reasonable and then addresses a problem which is associated with pressing of veneer. It describes how pine wood veneer, which is pressure-treated in a mechanical surface press or roller press, tends to be crushed and pulverized when the surface pressure exceeds 350 bar. This phenomenon, which of course is undesired, is due to the fact that the pressure is applied in one direction only.
Also SE 432 903 relates to a method for hardening wood by compressing flat wood elements. When carrying out the method, the wood element is placed in a treatment space, between two press devices which are movable relative to each other. Between the wood element and one of the press devices, there is further placed an elastic material layer, made of plastic or rubber. During the treatment, the wood element is compressed in one single treatment step by moving the press devices against each other to a desired mutual distance and thereafter moving them away from each other. During the compression, hard twigs force their way out of the wood element and into the elastic material layer, which counteracts splitting of the hard twigs. The treatment is to result in a permanent compression of the wood element without twigs being crushed, thus without deteriorating the quality of the treatment element.
However, it has proved that also this method results in a varying increase of the hardness in different parts of the treated element. Admittedly, the device for carrying out the method comprises, in addition to the two press devices, also two longitudinal side limiting strips. The task of these strips is probably to prevent the wood element from moving laterally during the treatment. In addition, the strips probably, to a certain extent, prevent the wood element from moving out laterally during the compression. Still, the fact remains that also this press device is only able to generate a pressure in one direction. As indicated in SE 432 903, this entails a limitation since the treatment pressure according to the document should not exceed 50 MPa or 500 bar. For pine wood, the pressure should not exceed 400 bar, which corresponds well to the problem mentioned in 7805483-0, that is, that pine wood veneer tends to be crushed when the treatment pressure exceeds 350 bar.
The devices described above for pressure treatment of wood thus all suffer from two serious defects. On the one hand, pressure treatment by means of these devices leads to a treatment result which varies over the surface of the treated element, and, on the other hand, the devices entail a limitation with respect to a relatively low maximum pressure, which can be used without damaging the wood to be treated.
The latter limitation is particularly serious since it has been found recently that higher treatment pressures, if they do not damage the wood, lead to a considerably better treatment result as regards hardening and compression stability.
GB 100,792 describes a method for pressure treatment of wood in which the treated wood is placed in a pressure medium and is subjected to a multilateral pressure, which is transferred to the wood via the pressure medium. The multilaterally applied pressure reduces the risk of crushing the pores of the wood during the treatment. For the method to function, it is required that no gas or liquid, which may be accommodated in the pressure medium, is allowed to penetrate into the wood during the pressure treatment. For that reason, the method is carried out with a specially viscous pressure medium, which is completely free from gases. Alternatively, the wood to be treated may be enclosed in an elastic material which is completely impenetrable to gas. A further condition for the method to function is that the pressure treatment is carried out at an elevated temperature which is above 90.degree. C. To this end, special heating members are arranged around the pressure chamber.
Although the method described in GB 100,792 entails a smaller risk of the pores of the wood being damaged during the treatment, it also has a number of disadvantages. For example, the method only permits the wood to be pressurized to a pressure of about 200 bar. In addition, it is required that the pressure treatment proceeds for a considerable period of time of about 2 to 3 hours. Further, the method also makes very special demands on the pressure medium being used, since this should be completely free from gas or liquid which may penetrate into the wood. Perhaps a still more serious limitation of the described method is that it requires special heating means, since the pressure treatment cannot be carried out at normal room temperature.
The object of the present invention is therefore to provide a device for pressure treatment of wood, by means of which the pressure treatment of the wood can be carried out with a satisfactory result in a considerably shorter time, whereby the wood can be pressurized at normal room temperature to pressures of more than 800 bar.